The present invention pertains to the field of surface acoustic wave (SAW) resonator filters.
Analyses of two-section SAW resonator filters with folded acoustic coupling have shown that these filters can provide an attractive combinations of filter characteristics. For example, the intercavity coupling level can be adjusted over a wide range to provide a wide range of filter bandwidths and proper design of an included U-path filter can provide a large out-of-band rejection.
The inherent temperature stability of these folded filters has been the subject of some concern because of the nonzero temperature coefficient of delay (TCD) path contained within the intercavity coupler, for example, see an article entitled "Monolithic Transversely Coupled SAW Resonator Filters," 1977 Ultrasonics Symposium Proc., IEEE Pub., 77 CH 1264-1 SU, pp. 888-893, by L. A. Coldren, R. L. Rosenberg, and J. A. Rentschler. For a number of SAW applications, the inherent temperature stability offered by the commonly used ST-cut X-propagating quartz substrate is not good enough. In particular, analyses of high Q-SAW resonators show this to be particularly true in oscillator applications where a filter phase shift of 1.degree. for a temperature shift of 1.degree. C. is not unusual.
Furthermore, two-section folded filters have not been considered extensively for use in oscillator applications. They appear to have a number of advantages over one-section devices, for example, a .+-.90.degree. tuning range is available over the 3-dB passband, the phase slope is doubled for a given bandwidth, the passband can be flat topped and better out-of-band rejection is possible in some configurations. However, as explained above with regard to filter applications, temperature stability is a problem.